On the official Bitcoin Forum, a tread on one of the mining pools I use, turned to a discussion about Bitcoin mining fees. Contrary to what many in the mainstream media believe, transaction fees are incurred when transferring Bitcoin.
However, these fees are very small compared to fees incurred by credit card networks and other fees most banks charge for transferring funds. Therefore, if you mine for Bitcoin, the best option is to set your payout minimums to the highest you possibly can since the smaller transfers tend to have a higher percentage of their value lost in transaction fees.
In addition to the discussion thread mentioned above, a website called BitcoinFees.com does thorough job of explaining how Bitcoin transaction fees work.